eg: tapeworm => cestode; flatworm => trematode; flukes, flagellates, hemamita => protozoan; nematodes are roundworms, etc.

That way when you're cross referencing drugs vs bugs, you can keep your sanity.  Or lose less of it anyway.

Human and livestock medical info seems to be a pretty good source of info about what these drugs are effective against.  For example, praziquantel is primarily for flatworms and relatives (trematodes), and protozoa (including flagellates) (cestodes).

Metronidazole seems to have several properties including antibiotic and anti-protozoa (cestode).  I believe its the drug-of-choice for the heximita (hole-in-the-head) protozoa (cestode).  The fact that it has antibiotic properties would make it extra useful in fish worming medications to reduce the risk of internal infections after any intestinal worms detach.

Levamisole is also an anti-cancer drug in humans as well as being used as an immune system stimulant.  In humans, it is sometimes used alongside praziquantel for the stimulation effect to enable praziquantel to work better.

For worms, Levamisole is a paralyzing agent.  They are often expelled alive.. hence the need to vacuum them up. (eep! I wasn't aware of this! See http://www.elephantcare.org/Drugs/levamis.htm under Pharmacology)

Praziquantel works by disturbing the worm's internal membranes.  They kinda self destruct by leaking into themselves.  Roundworms don't have the same membranes.

Another example.. a cat worming site:
"Some worming medications are effective against both roundworm and tapeworm, while others are only effective against one or the other."

"Drontal cat TM (Bayer)
Contains pyrantel (active against roundworm) and praziquantel (active against tapeworm), in a tablet form."

There's another reference for praziquantel not being effective against for roundworms (implicitly nematodes).  But that wouldn't show up on a search for camallanus or nematodes.